The present invention relates broadly to a radar apparatus, and in particular to a phase/phase/frequency-scan radar apparatus.
In the prior art, the need for high speed surveillance radar is well recognized. The present day and future functional requirements for military surveillance radars are dominated by the need to provide target search and track functions over large coverage volumes in environments that include many high speed and maneuvering targets, with an associated need for track information to be generated on newly detected targets in the shortest possible track generation time. Radar units which employ antennas with two-dimensional agile beam capabilities are best suited for these purposes, and stationary phase/phase scanned antennas have been used to provide such capabilities in some well known present day systems, such as Patriot and Aegis. However, phase/phase scanned antennas are very costly, often prohibitively so, when it is required to provide 360.degree. coverage in azimuth. Also, in order to achieve this 360.degree. of coverage in azimuth, it is necessary to employ a minimum of three, and typically four, antenna arrays.
The state of the art of surveillance radar is well represented and alleviated to some degree by the prior art apparatus and approaches which are contained in the following U.S. Patents:
U.S. Pat. No. 3,646,559 issued to Wiley on Feb. 29, 1972; PA1 U.S. Pat. No. 3,860,928 issued to Ehrlich on Jan. 14, 1975; PA1 U.S. Pat. No. 4,724,438 issued to Arnold et al on Feb. 9, 1988; and PA1 U.S. Pat. No. 4,779,097 issued to Morchin on Oct. 18, 1988.
The Wiley patent describes a multimode antenna for simultaneously providing at least one frequency-scanning mode and a phase-scanned mode. The isolated ports of the directionally coupled radiating apertures of a cross-fed frequency-scanned array, are adapted to be fed from a fixed frequency source by a phased array of voltage-controlled phase shifters or by mechanically scanned means for adjusting the phase gradient (or by a combination of the two). The fixed frequency corresponds to a direction-frequency (of the frequency-scanned array) outside the range of direction to be covered by the phased array, thereby providing mutual isolation between the phased-array energy and the frequency-scanning energy.
The Ehrlich patent is directed to a system of radiating elements arranged for forming one or more beams of radiation having radiation patterns such as a monopole, dipole, quadrupole, other multipoles or combination thereof. The individual radiating elements of the array are interconnected by circuitry providing for the summing and differencing of signals provided by adjacent radiating elements in response to incident radiation.
The Arnold et al patent relates to a radar system of the type which can be presented with a number of tasks to be performed. Some of these tasks, e.g. the surveillance of areas at close range, may only require the transmission of low energy pulses and thus the full potential of the r.f. energy source has not previously been used while such tasks are being handled.
The Morchin patent discusses a segmented phased array antenna system for scanning two different ranges of directions with a single set of antenna elements which are movable between first and second positions. In each set of positions, the antenna elements are operated as a conventional phased array radar system.
A more cost-effective approach that is currently being considered for future systems, such as the Advanced Tactical Surveillance Radar for the USAF, is to employ a single rotating agile beam array, but with provision for operation in a stationary, trainable mode when the threats are predominantly in a limited azimuth sector. This type of operation, particularly in the rotational mode, imposes severe time/energy management demands on the radar, and generally requires the provision of multiple simultaneous beams to accomplish the search function. On the other hand, the track function requires individual steerable pencil beams.
While the above-cited references are instructive, a need remains to provide a surveillance radar to accommodate threats in a limited azimuth sector. The present invention is intended to satisfy that need.